The present disclosure relates to a twin roll strip casting method, and more particularly, to a twin roll strip casting method for producing strips having a high degree of edge quality.
In general, strips are produced as follows. A slab is produced from molten steel through a continuous casting process, and the slab is formed as a hot-rolled coil through a reheating and hot-rolling process. The hot-rolled coil undergoes hot band annealing and pickling in a hot annealing & pickling line, and then the hot-rolled coil is formed as a final product through a cold rolling process.
The above-mentioned continuous casting method is complex because casting and rolling processes are performed separately. In addition, since reheating slab is necessary to allow the rolling process to be performed after the casting process, a large amount of energy is consumed, and thus the continuous casting method is not favored in terms of economical and environmental aspects. In order to overcome demerits of the continuous casting method, a twin roll strip casting method in which a thin steel sheet (strip) is produced directly using twin rolls has been developed.
FIG. 1 is a schematic view illustrating a twin roll strip casting process of the related art.
In a twin roll strip casting method of the related art, as shown in FIG. 1, a pair of internal water-cooled rolls 110 are rapidly rotated in mutually engaging directions, and while molten steel 130 is supplied to a region between the rolls 110 through an injection nozzle 120, a strip 140 having a thickness of 10 mm or less is drawn out therefrom.
When strips are produced by such a twin roll strip casting method, defects such as edge bulges or edge cracks may be formed. Edge bulges make it difficult to perform a hot rolling process and cause edge cracks or shape errors during a rolling process, thereby decreasing the process yield.
If 200 mm to 300 mm wide edge regions of a strip are relatively hot, as compared to the other regions of the strip, due to an edge region solidification delay phenomenon, the thickness of the strip may be locally increased (to about 50 mm) in the edge regions, or the strip may be torn. This is a phenomenon is known as “edge bulging.” The edge region solidification delay phenomenon may be considered to be a characteristic of a twin roll strip casting process. When shells undergo solidification and shrinkage, if edge regions of the shells that are relatively free, as compared to center regions of the shells, are separated from casting rolls, edge region solidification delay may occur due to a relatively poor heat transfer rate in the separated edge regions. If the edge region solidification delay occurs severely, edge bulging may occur. In this case, the edge regions may crack or molten steel may leak during a casting process. Furthermore, during a rolling process, a reduction ratio may be varied in the width of a strip as the edge regions may be relatively thick or hot. Therefore, the shape of the strip may be deformed, or due to poor rolling conditions caused by edge waves, cracks may be formed, thereby lowering the process yield.